1. Field of the Invention
The present invention relates to a passive restraint, such as an air bag unit, used for protecting a driver and passengers in a vehicle when the vehicle crashes. More specifically, the invention pertains to an apparatus for controlling activation of such a passive restraint as well as to a method of the same.
2. Description of the Prior Art
One example of known apparatuses for controlling activation of passive restraints is an apparatus for controlling ignition of a squib included in an air bag unit. In the air bag unit, a gas-generating agent is ignited with a squib in an inflator to evolve a gas from the inflator, and a bag is then inflated with the gas in order to protect a driver or a passenger from the impact of collision.
Conventional apparatuses for controlling ignition of squibs in such air bag units generally measure an acceleration applied to the vehicle and carries out the ignition control of the squibs based on the measured acceleration. The acceleration applied to the vehicle that is in a collision, however, often includes not only a longitudinal component along the longitudinal axis of the vehicle but a lateral component along the lateral axis of the vehicle. It is accordingly more effective to control ignition of squibs based on both the measured accelerations acting along the longitudinal axis and the lateral axis of the vehicle.
A proposed activation control apparatus, for example, as disclosed in JAPANESE PATENT LAIED-OPEN GAZETTE No. 6-56000, controls ignition of squibs based on measured accelerations acting along the longitudinal axis and the lateral axis of a vehicle.
The prior art apparatus specified above has a longitudinal acceleration sensor (longitudinal G sensor) for measuring an acceleration along the longitudinal axis of a vehicle and a lateral acceleration sensor (lateral G sensor) for measuring an acceleration along the lateral axis of the vehicle. The prior art apparatus first calculates an arithmetic value fx for the longitudinal direction (direction X) from the acceleration in the longitudinal direction and an arithmetic value fy for the lateral direction (direction Y) from the acceleration in the lateral direction. The apparatus then derives a composite vector F based on these arithmetic values, and calculates a magnitude f and a direction .theta. of the composite vector F according to Equations (1) and (2) given below, wherein the direction .theta. is given as an angle relative to 0 degree, which represents the front center of the vehicle along the longitudinal axis thereof: ##EQU1##
The magnitude f of the composite vector F is compared with a threshold value fTh (.theta.) previously set for the direction .theta.. When the magnitude f is greater than the threshold value fTh(.theta.), the gas-generating agent is ignited with squibs.
This prior art apparatus controls the ignition of squibs and thereby controls the activation of air bag units by taking into account the magnitude of the acceleration applied to the vehicle as well as the direction of the acceleration. This structure realizes appropriate activation control, irrespective of the direction of the acceleration applied to the vehicle.
In the proposed apparatus, whenever some acceleration is applied to the vehicle, the magnitude f and the direction .theta. of the composite vector F are calculated according to Equations (1) and (2) given above. These arithmetic operations, however, include rather complicated and time-consuming calculations, such as second power, square root, division, and arc tangent. This means that rather complicated and time-consuming arithmetic operations are required to determine the magnitude f and the direction .theta. of the composite vector F. Equation (2) includes a division of fy/fx. When fx is approximately equal to zero, the value of the direction .theta. diverges and can not be determined.
The threshold value fTh(.theta.) should be set for the direction .theta., in order to determine whether or not ignition with squibs is to be carried out. For the determination of the threshold value fTh(.theta.), it is required to set appropriate equations and execute complicated and time-consuming arithmetic operations.